Accurate determination of the photoluminescence (PL) spectrum of molecules is crucial in many areas including molecular diagnostics, genome analysis, and organic electronics. However, interpretation of experimental spectra from thin films can be complicated by phenomena extrinsic to the site of photon emission such as optical interference and self-absorption. In this study, the alteration of fluorescence spectral lineshape is modeled from a thin film due to extrinsic factors as function of film thickness, incident angle, bandgap, and Stokes shift. The method is experimentally verified for thin (<400 nm) layers of molecular semiconductors on quartz substrates. Finally, a chart is provided for estimating the magnitude of the error due to extrinsic factors in the bandshape of fluorescence spectra from thin films. This demonstrates that the 0-0/0-1 intensity ratio, an indicator of the nature of excitonic coupling, is especially sensitive to possibly unconsidered influence from extrinsic effects.